PM should have sought parliamentary approval before any military strike, opposition Labour Party leader says.

The leader of Britain's largest opposition party is suggesting Theresa May, the prime minister, could face a backlash in parliament for her decision to join the US and France in launching strikes against Syria.

"May should have sought parliamentary approval, not trailed after Donald Trump," he said on Saturday.

Corbyn said the strikes will make assigning blame for the use of chemical weapons in Syria "less, not more, likely".

"Bombs won't save lives or bring about peace," he said, adding that Britain should be leading the response and "not taking instructions from Washington and putting British military personnel in harm's way".

Many politicians, including some in May's own Conservative Party, had backed his call for parliament to be asked before any military strike.

Assurance sought

Corbyn later wrote to May seeking assurance that there would be no further bombing raids and urged the government to negotiate a pause in the Syrian civil war.

The action following an alleged chemical-weapons attack on the rebel-held town of Douma on April 7 was condemned by several British opposition parties, who wanted parliament recalled.

May will appear before the House of Commons on Monday to explain her decision on joining the strikes.

The prime minister's office said she had spoken to Mohammed bin Salman, the crown prince of Saudi Arabia; Turkish President Tayyip Erdogan; German Chancellor Angela Merkel and the prime ministers of Italy, Australia and Canada about the strikes.

It said they all agreed with her on "the importance of restoring the international norm that the use of chemical weapons is never acceptable".

The small Northern Irish political party that props up her government said May was justified in taking such action.

Stop the War, a pacifist coalition once chaired by Corbyn, has called a demonstration outside parliament in London on Monday to protest against the strikes.

The group said it "strongly condemned" the action and accused May of "sanctioning killing" at US President Donald Trump's behest.

Often when the British government decides on military action, the opposition offers its full support.

However, that has been less the case in recent years.

David Cameron, May's predecessor, lost a parliamentary vote on air strikes against Assad's forces in 2013 when 30 Conservative politicians voted against action, with many Britons wary of entering another conflict after interventions in Iraq, Afghanistan and Libya failed to bring stability to the region.

Legislators backed action in Iraq in 2014, and again in Syria in 2015, strictly limiting strikes in both countries to targets of the Islamic State of Iraq and the Levant (ISIL) group.

A BMG poll, taken before the strikes and published by the Independent newspaper on Saturday, indicated that 28 percent of Britons backed air strikes, with 36 percent opposed.